Entanglement of two superconducting qubits in a waveguide cavity via monochromatic two-photon excitation

TL;DR

This paper demonstrates a method for entangling two superconducting qubits in a microwave cavity using monochromatic two-photon excitation, enabling all-microwave two-qubit control with high fidelity.

Contribution

It introduces a novel two-photon excitation technique to generate Bell states in superconducting qubits within a cavity, achieving high-fidelity entanglement.

Findings

Achieved 90% two-qubit gate fidelity.

Demonstrated Ramsey and spin echo sequences with Bell states.

Enabled all-microwave two-qubit control in superconducting circuits.

Abstract

We report a system where fixed interactions between non-computational levels make bright the otherwise forbidden two-photon 00 --> 11 transition. The system is formed by hand selection and assembly of two discrete component transmon-style superconducting qubits inside a rectangular microwave cavity. The application of a monochromatic drive tuned to this transition induces two-photon Rabi-like oscillations between the ground and doubly-excited states via the Bell basis. The system therefore allows all-microwave two-qubit universal control with the same techniques and hardware required for single qubit control. We report Ramsey-like and spin echo sequences with the generated Bell states, and measure a two-qubit gate fidelity of 90% (unconstrained) and 86% (maximum likelihood estimator).